1. Field of the Invention
The present invention relates to a capacitor used for storing the electrical charge of a memory cell and the like in a DRAM (Dynamic Random Access Memory), for example, and to a method for manufacturing the same.
2. Description of the Related Art
There is a type of capacitors as stacked capacitors whose capacitive insulating film is formed with tantalum oxide (Ta2O5) which exhibits a high dielectric constant and withstand voltage. Amorphous silicon is formed in an inner wall of an opening part of an interlayer insulating film. It is then made to be polycrystalline and conductive by a heat treatment thereby forming a lower electrode. A silicon nitride film (SiN) is formed on the surface of the polysilicon lower electrode. Furthermore, a capacitive insulating film of tantalum oxide is formed thereby forming an upper electrode of titanium nitride film (TiN).
In order to increase the effective electrode surface area for increasing the electrostatic capacity, an HSG (Hemi Spherical Grain) nucleus formed with a group of semispherical crystal grains is formed on the surface of the amorphous silicon. That is, the oxide film on the surface of the amorphous silicon as the base of the lower electrode is removed for forming the uneven surface. The more amorphous silicon is deposited on the uneven surface part by chemical vapor deposition (CVD) thereby forming the lower electrode. Then, it is cleaned with a chemical solution for removing the oxide film and forming the HSG (Hemi Spherical Grain) nucleus constituted of a group of semispherical grains. Then, a heat treatment is performed for forming HSG-Si. Subsequently, the silicon nitride film, the capacitive insulating film made of tantalum oxide, and upper electrode of the titanium nitride as described above are formed thereon.
There are following shortcomings in the capacitor made of tantalum oxide. Electric fields are locally concentrated in the particles or in the projected part of grain boundaries of HSG-Si, which are generated when forming HSG-Si, so that TDDB (Time Dependent Dielectric Breakdown) characteristic is deteriorated. Furthermore, with tantalum oxide, the potential barrier becomes small so that a large leak current is caused.
Further, in the process between the forming the lower electrode to the wiring, there may use reducing gas in hydrogen sintering (heat treatment) or when forming a film by NH3 gas. The tantalum oxide film is reduced to metallic tantalum by the reducing gas and the TDDB characteristic is deteriorated.
Moreover, in the wiring step and the like after forming the upper electrode, plasma processing may be performed for forming an interlayer insulating film and a contact layer in contact. By the plasma processing, the tantalum oxide film is damaged and the TDDB characteristic is deteriorated.